Well, the comparison with Castle was only in regards to watchability and overall genre. I think it would be doing the show a disservice to come to it expecting another Castle. It's a very British show, quite non-violent, and based more on dialog than action. In that regard it's a lot more like some of the other major British cop shows, like for example Midsomer Murders, but with a bit more humor injected. Character based humor rather than laugh-out-loud comedy.
The premise is the creation of UCOS, Unsolved Crimes and Open case Squad, with three retired police detectives and their female boss. In the first eight seasons they are played by Alun Armstrong, James Bolan, Dennis Waterman and Amanda Redman.
Youtube is definitely not the ideal way to watch shows (or movies), but several episodes are available there, and you might check out one or two to see if it is something you might like.